1. Field of the Invention
The present invention relates generally to the replacement of a natural kneejoint with a knee prosthesis and pertains, more specifically, to achieving an extended range of motion with reduced contact stresses within the components of the knee prosthesis and concomitant improved articular performance, together with greater longevity.
During articulation of a natural kneejoint, flexion between the tibia and the femur takes place about a transverse axis while, at the same time, some relative rotation between the tibia and the femur occurs about a longitudinal axis. Such flexion and rotation is necessary to carry out a normal gate cycle. In addition, the femur experiences posterior rollback, that is, a combination of backward rolling and sliding of the distal femur along the proximal tibia as the knee flexes. Stability of the knee, during flexion and extension, depends, in part, upon the action of the cruciate ligaments and the collateral ligaments. During extension, the collateral ligaments tighten, while during flexion the collateral ligaments loosen to enable articulation of the femur and tibia to be accompanied by posterior rollback and internal tibia rotation, allowing deep flexion of the knee joint, that is, flexion in the range of approximately 110° to 150° of flexion.
2. Description of the Related Prior Art
Currently available knee prostheses utilize femoral components having posterior condyles which provide a maximum coverage of the distal femur. Rather than mimicking the natural posterior condyle shape and length, the posterior condyles of a femoral component of such knee prostheses is lengthened relative to the length of the natural posterior condyles. The extended length often prevents loosening of the collateral ligaments, as seen in the natural knee during flexion, resulting in a tight knee with a reduced range of flexion.
Additionally, current knee prostheses do not allow for anatomically accurate rotation about the longitudinal axis in deep flexion. In deep flexion, the condylar surfaces of the femoral component are forced against the counterpart articular surfaces of the tibial component, lifting the femoral component from the tibial component as rotational forces are increased, with the result that the knee becomes distracted and excessive wear is generated in the tibial component. Further, by virtue of the relative contours of the condylar surfaces and the articular surfaces, the posterior condyles of current knee prostheses operate to reduce contact area in deep flexion, causing increased contact stress with concomitant increased wear in the tibial component. Similarly, the relative contours of the condylar surfaces and the articular surfaces tend to reduce the range of hyperextension, tend to cause distraction of the knee as hyperextension is increased, and engenders higher contact stress as hyperextension is increased, resulting in excessive wear in the tibial component.